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gmat prep problem

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by dbart06 » Sun Jun 22, 2008 6:32 am
HI! new to forum.

this problem has symbols. The problem reads:

for all positive integers, M=3m when m is odd, M=1/2m when m is even, which is equivalent to 9*6. remember M has a square around it as well as the 9 & 6. sorry, i wasnt able to screen shot. any help would be appreciated.
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by Nycgrl » Sun Jun 22, 2008 1:59 pm
can you please restate the problem

U can use ^ for square like for 3sqr 3^ 2

It will make it easy to understand.

Thanks
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by dbart06 » Sun Jun 22, 2008 3:35 pm
for all positive integers, [M]=3m when m is odd, [M]=1/2m when m is even, which is equivalent to [9]*[6].

a)81
b)54
c)37
d)27
e)18

sorry for the confusion.
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by Nycgrl » Sun Jun 22, 2008 4:21 pm
No problems even I had problems with signs when I was new

Is it B???
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by dbart06 » Sun Jun 22, 2008 4:27 pm
oa = d 27
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by Ian Stewart » Mon Jun 23, 2008 5:33 am
dbart06 wrote:for all positive integers, [M]=3m when m is odd, [M]=1/2m when m is even, which is equivalent to [9]*[6].

a)81
b)54
c)37
d)27
e)18

sorry for the confusion.
That's not quite what the question says, if I remember this correctly from GMATPrep. The answer choices should be:

a) [81]
b) [54]
c) [37]
d) [27]
e) [18]

We know [m]=3m when m is odd, [m]=(1/2)*m when m is even. Thus:
[9] = 3*9 = 27
[6] = 6/2 = 3

and [9]*[6] = 27*3 = 81. This often leads people to choose answer choice A, but notice that answer choice A is not equal to 81. The [] function is applied to each answer as well; answer choice A is [81] = 3*81 = 243. D is the correct answer, because [27] = 3*27 = 81 = [9]*[6].
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by dbart06 » Mon Jun 23, 2008 6:53 am
Thanks so much! It can be frustrating to be told you answered a question wrong with no explanation.
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by medea66 » Wed Aug 20, 2008 6:20 pm
I just came across this problem in GPREP1 as well, and yes Ian, these guys are really out to get us.
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by pre-gmat » Wed Aug 20, 2008 7:51 pm
I think the problem can be solved much easily.

[9]*[6] is essentially [54] and this is an even number so as per the question M=1/2 m when m is even.

1/2 *[54] =27

Let me know if my reasoning is wrong.
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by Ian Stewart » Thu Aug 21, 2008 4:12 am
pre-gmat wrote:I think the problem can be solved much easily.

[9]*[6] is essentially [54] and this is an even number so as per the question M=1/2 m when m is even.

1/2 *[54] =27

Let me know if my reasoning is wrong.
You cannot multiply through functions in the way you're attempting to do- that is, f(x)*f(y) is not normally equal to f(x*y). [9]*[6] is not equal to [54], and is not equal to 27; as described above, it is equal to [27], or 81.
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by pre-gmat » Thu Aug 21, 2008 6:53 am
Thanks for making the concept clear.
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by 650 or bust » Sat Nov 08, 2008 1:48 pm
Hello,

I'm new to the forum, too. I have a quick question...

I got to the point that 81 was the answer, but... how do we know when to stop? [81]=3m and m=27. How do we know that the GMAT testers aren't carrying it again? Or throwing in another trick?

Any ideas?

By the way, the explanations on this forum are EXCELLENT! Thank you!
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by Abdulla » Sat Nov 08, 2008 8:18 pm
I fell in the same trap today.. :(
Abdulla
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by ironsferri » Sat Jul 18, 2009 1:35 pm
Hi everyone,

I don't quite get why to substitute 9 and 6 in the m, and not solve for m.
In specific,

[9] = 3*9 = 27
[6] = 6/2 = 3

I think [9] goes where [M] is, and not also in place of m. Therefore

[9] = 3*m
m = 3

The same with [6]

[6] = m*/2
m = 12

Can somebody advise?

Thank you so much
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